1. Field of the Invention
The present invention relates to a dip coating method and a dip coating apparatus, and in particular, to a dip coating method and a dip coating apparatus which are suited to the coating/formation of a photosensitive layer on the outer peripheral surface of a cylindrical member in the manufacturing of an electrophotographic photoreceptor.
2. Description of the Related Art
Conventionally, inorganic compounds such as selenium, cadmium sulfide, and zinc oxide, and organic compounds exemplified by polyvinyl carbazole have been proposed as photoconductive materials for forming the photosensitive layer of an electrophotographic photoreceptor. Further, in multi-layered-type electrophotographic photoreceptors in which the photosensitive layer is divided into a charge-generating layer and a charge-transporting layer, various organic compounds have been proposed as the charge-generating material and the charge-transporting material and have been used as organic photoreceptors.
The following conventional coating methods have been known as methods for manufacturing such organic photoreceptors: a dip coating method, a spray coating method, a spin coating method, a bead coating method, a wire bar coating method, a blade coating method, a roller coating method, an extrusion coating method, a curtain coating method, and the like. The dip coating method has been widely used in particular as a method for forming a uniform photosensitive layer on the outer peripheral surface of a cylindrical member.
A plural-member simultaneous dip coating method, in which a plurality of cylindrical members are dipped in a coating solution and are simultaneously raised up out of the coating solution, is generally used in the dip coating of an electrophotographic photoreceptor, from the standpoint of improving production. Therefore, in a dip coating method utilizing a coating solution circulating mechanism, a large amount of coating solution is needed for circulation. When a coating solution having a short pot life is used, the amount of coating solution which is disposed of in a short period of time is, of necessity, great, and the efficiency with which materials are used thus deteriorates. Generally, when the dip coating method is used, the pot life of the coating solution is important. However, occasions often arise in which materials having short pot lives must be used due to the characteristics required of the photoreceptor.
With the dip coating method, one reason why the pot life of the coating solution is short is because it is easy for the coating solution to contact the outside air when materials are used for which a hydrolysis reaction occurs easily due to an oxidation reaction or absorption of moisture in the air, and thus, the coating solution deteriorates. In order to prevent deterioration of the coating solution in, in particular, a dip coating method utilizing a coating solution circulating mechanism, it is necessary to restrict the contact of the coating solution with outside air at an overflow surface at the coating solution tank, or at a solution receiving tank which recovers the coating solution which has overflowed from the coating solution tank, or the like.
A conventional apparatus has been proposed in which a cover is provided above the overflow surface as a means for restricting the contact of the coating solution and the outside air. Because the cylindrical member is always dipped in the coating solution, the opening portion, which is formed in the cover and through which the cylindrical member can pass, is made small. A shutter or the like is provided at the opening portion as a means for closing the opening portion at times when no cylindrical member is being dipped in the coating solution. However, a drawback arises in that such a apparatus is complicated.
Further, a structure has been proposed in which a hood is provided, and a means for making the solvent gas, which evaporates from the coating solution, stay at the hood portion is provided (Japanese Patent Application Laid-Open (JP-A) No. 59-30662). In JP-A-59-90662, a plural-level structure is provided in which shielding plates are provided at the inner portion of the hood, and an opening is formed in the wall surface of each level. The solvent gas concentration gradient is provided by the shielding plates in the inner side of the hood such that sudden drying of the coated film is suppressed.
However, in this method, opening portions for providing a concentration gradient are provided at each level. Therefore, when the cylindrical member is dipped in the coating solution, the substitution of the outside air and the solvent gas which is staying at each level is promoted, which is disadvantageous from the standpoint of suppressing contact of the outside air and the coating solution. When suppression of contact of the coating solution with the outside air is realized, the effects of such suppression cannot be expected if the height of the hood is not great enough and the hood diameter is not made as small as possible. However, when the hood diameter is made small and the height of the hood is made large, the drying speed of the coated film is too slow, and a drawback arises in that the characteristics of the formed film deteriorate such as the thickness of the film coated on the surface of the cylindrical member is not uniform, i.e., the so-called sagging phenomenon is marked.
The coating solution tank disclosed in Japanese Utility Model Application Laid-Open (JP-U) No. 62-2053, which does not have a coating solution circulating mechanism, aims to prevent the adhesion of solid materials to the upper portion of the coating solution tank which adhesion is caused by the drying of the coating solution due to the surface of the solution fluctuating up and down due to the coating operation. A baffle plate having a plural-level structure is provided within the coating solution tank. When the surface of the solution falls at the time the cylindrical members are pulled up, the solvent vapor stays at the top portion of the coating tank such that drying is suppressed. However, when the cylindrical members are dipped in the coating solution, the surface of the solution rises. Therefore, at the space at the upper portion of the coating solution tank, there is no substitution of the outside air and the solvent vapor, and there is an extreme reduction in sagging.
The suppression of the contact of the coating solution and the outside air leads to the concentration of the solvent gas in the coating drying zone becoming greater, and as a result, sagging increases. Accordingly, it is desired to suppress the contact between the coating solution and the outside air and to suppress sagging as much as possible.